Steels used for structural and wear-resistant applications include, for example, high manganese steels and certain medium carbon steels with or without the hardening and strengthening elements chromium, nickel or molybdenum--such as SAE 4140, SAE 3140 and SAE 1345. High Manganese alloy steels are expensive and require complicated heat treatment to develop required properties. For example, such steels commonly are reheated, for example to around 1100.degree. C., after hot working and then water quenched to form austenite. Heat treatment of SAE 3140, SAE 4140 and SAE 1345 steels also is complicated, requiring oil quenching and high temperature tempering. The strength, toughness and wear-resistance properties of the less expensive steels such as SAE 1345 are quite low.
Such shortcomings of prior art steels were partially overcome by certain medium carbon and medium-high carbon, manganese-boron bainite steels as described in Chinese patent application numbers 86103008 and 87100365. Such steels, having a duplex bainite-martensite structure after air-cooling, are simply processed, have low cost and good strength, toughness and wear resistance. However, such steels have relatively low hardenability after air-cooling. For example, they are hardenable by air-cooling to a hardenable diameter of only about 20 mm. Within such limits, these steels are useful in a forged or rolled condition; they are not useful for application as castings of larger section thicknesses. Attention also is directed to certain low carbon, Mn--Si--B steels, having a principally bainitic structure, as disclosed in U.S. patent application Ser. No. 083,130. Use of such steels provides full section hardenability of bars with a cross-section diameter of at least 30 mm.
The term "hardenable diameter" is commonly used to described the maximum depth dimension throughout which an article is hardenable to a particular hardness level. This term refers to the diameter of a test specimen, normally in the form of a rod or bar having a uniform cross-section normal to the specimen length.
The compositions of such prior art steels, in weight percent, are given in Table 1.
TABLE 1 __________________________________________________________________________ C Mn Si Cr Ni Mo B __________________________________________________________________________ High Manganese 1.0 11 0.3 Steel to 1.4% to 14% to 0.9% SAE 3140 0.37 0.5 0.2 0.45 1.0 to 0.44% to 0.8% to 0.4% to 0.75% to 1.4% SAE 4140 0.38 0.5 0.2 0.9 0.15 to 0.45% to 0.8% to 0.4% to 1.2% to 0.25% SAE 1345 0.42 1.4 0.2 to 0.49% to 1.8% to 0.4% Chinese Appln. 0.31 2.1 0.1 0.0005 No. 86103008 to 0.46% to 3.4% to 1.5% to 0.005% Chinese Appln. 0.47 2.1 0.1 0.0005 No. 87100365 to 0.60% to 3.5% to 1.5% to 0.005% U.S. Appln. 0.10 2.0 0.3 No. 083,130 to 0.25% to 3.2% to 1.5% __________________________________________________________________________
The last three Table 1 steels optionally may contain up to 1.5% of tungsten or chromium, up to 1% molybdenum and up to 0.15% vanadium.